When it is desired to do work on a submerged object whose location is unknown, with a movable water-borne structure, the common present practice is to use divers to first locate the object and then to maneuver the structure about the object. For example, the structure can be an underwater pipeline trenching machine mounted on a sled which is towed by a surface craft. The trenching machine digs a trench and allows the pipeline to settle at the bottom of the trench, which is subsequently covered with material from the ocean floor. The present practice is to send divers down to the seabottom who by voice communication first assist the surface craft in setting astride the sled over the pipeline and then assist the craft in guiding the sled along the path of the pipeline as the digging of the trench progresses.
Since the present tendency is to bury pipelines at deeper and deeper water depths, the divers will become exposed to greater and greater water pressures. Also, in relatively shallow waters, any turbidity or murkiness in the water greatly reduces a diver's field of vision, say to a few inches. Attempts have been made to use underwater television cameras with diver assistance, but the range of vision of such cameras can also become very limited.
Accordingly, it is a broad object of this invention to provide, by means of dual sonar slant ranging, a continuous visual representation of the position of the sled with respect to the piepline and the seabottom, without diver assistance and/or use of television cameras.
Methods are generally known for acoustically surveying underwater. One such method employs dual side scan sonar which typically comprises five major components: a transceiver, a split-trace programmer, a graphic recorder, a tow cable, and a towed vehicle carrying two transducers.
The transceiver contains a transmitter as well as a receiver. The transmitter generates a short burst of electric energy which is carried to the transducers via the cable. Each transducer changes the burst of electric energy into a directional acoustic pulse which is radiated out into the water in a fan-shaped beam. The transmitted acoustic beams from the pair of transducers are consecutively directed downwardly but away from each other. Each acoustic beam is reflected from the seabottom and from any hard object lying in the path of the beam. The same transducer which emits the acoustic impulse is used to detect its reflected beam or echo and to convert it into an electric pulse which is transmitted via the tow cable back to the receiver section of the transceiver.
The programmer generates key pulses into the transceiver. The key pulses control the channels of transmission. The programmer also blanks out undesired signals and noise which are present on the channel which is not in use, so that when transmitting and receiving from one transducer, signals from the other transducer will not be recorded.
The tow cable combines tensile cables with a core of electric conductors coupling the transducers to the transceivers.
The recorder produces visual records of the sonar survey which are usually qualitative.
A commercially available dual side scan sonar system of the foregoing type is available from Ocean Research Equipment, Inc., of Falmouth, Mass.
It is another object of this invention to utilize the technology developed in connection with side scan sonar systems in a novel manner and for a novel use consisting of maneuvering underwater a structure relative to a stationary, sound-reflecting object.
It is a further object of this invention to provide a sonic navigation system for installation directly on a remotely-piloted, water-borne structure. The navigation system may be used (1) to locate the stationary object, such as a pipeline, at ranges up to 350 and more meters, (2) to orient and position the structure precisely with respect to the pipeline in both the vertical and horizontal directions, and (3) to guide the structure in its movement relative to the pipeline.